Field of Endeavor
The present invention relates to shape memory devices and more particularly to a bidirectional shape memory device.
State of Technology
Shape-memory materials have the useful ability of being formable into a primary shape, being reformable into a stable secondary shape, and then being controllably actuated to recover their primary shape. Both metal alloys and polymeric materials can have shape memory. In the case of metals, the shape-memory effect arises from thermally induced solid phase transformations in which the lattice structure of the atoms changes, resulting in macroscopic changes in modulus and dimensions. In the case of polymeric materials, the primary shape is obtained after processing and fixed by physical structures or chemical crosslinking. The secondary shape is obtained by deforming the material while is an elastomeric state and that shape is fixed in one of several ways including cooling the polymer below a crystalline, liquid crystalline, or glass transition temperature; by inducing additional covalent or ionic crosslinking, etc. While in the secondary shape some or all of the polymer chains are perturbed from their equilibrium random walk conformation, having a certain degree of bulk orientation. The oriented chains have a certain potential energy, due to their decreased entropy, which provides the driving force for the shape recovery. However, they do not spontaneously recovery due to either kinetic effects (if below their lower Tg) or physical restraints (physical or chemical crosslinks). Actuation then occurs for the recovery to the primary shape by removing that restraint, e.g. heating the polymer above its glass transition or melting temperature removing ionic or covalent crosslinks, etc. However, devices made from shape memory polymer are only able to recover in one direction, i.e. they can go from any shape back to their original shape, but have to be manually forced back to any other shape.
Types of Shape Memory Polymer
Thermoplastic SMPs—thermoplastic polymers are those which can be heated into a melt state in which all prior solid shape memory has been lost, processed into a shape, and solidified. If need be they can be re-heated to their melt state and re-processed a number of times. In thermoplastic SMPs, the shape memory effect generally relates to the material having a multiphase structure in which the different phases have different thermal transitions, which may be due to glass transitions, crystalline melting points, liquid crystal-solid transitions, ionomeric transitions, etc. The primary shape is obtained by processing in the melt state above the highest transition temperature and then cooling to a temperature in which either a hard phase or other physical crosslink is formed to lock in that shape. The secondary shape is obtained by bringing the material to a temperature above its actuation temperature but below its melting temperature, mechanically shaping the material into its secondary shape, then cooling it below its actuation temperature. Suitable thermoplastic SMPs include block copolymers (linear di, tri, and multiblocks; alternating graft), immiscible polymer blends (neat and with coupling agents such as di or tri-block copolymers), semi-crystalline polymers, and linear polymers with ionomeric groups along the chain or grafted to the chain.
Thermosetting SMPs—thermosetting polymers are those which are processed into a part and simultaneously chemically crosslinked, so that the part is essentially one macromolecule. They cannot be re-processed by melting. In thermosetting SMPs the primary shape is obtained during the initial processing step involving crosslinking. The secondary shape is obtained by mechanically reshaping the material at a temperature or condition in which the material is in an elastomeric state. This secondary shape is locked in by cooling the material below the actuation temperature, which relates to a transition as described above. Suitable thermosetting SMPs include all of the types of materials described under thermoplastic SMPs but which can also be chemically crosslinked to form the primary shape. In addition, crosslinked homopolymers can also be used as SMPs with the actuation temperature typically being the glass transition temperature of the material.